Black-and-white photographic prints or images are obtained generally by imagewise exposure of black-and-white photographic silver halide positive materials. The latent image is then processed using the appropriate photochemicals to provide the appropriate development of the silver and desilvering ("fixing") to remove unexposed silver.
Black-and-white photographic silver halide positive materials refers to those materials in which a positive black-and-white image can be obtained, including black-and-white reflective prints, black-and-white positive transparencies and black-and-white motion picture intermediate and print films. Negative materials refer to those materials in which a negative image is created that can then be used later to provide a positive viewing image. Such materials include black-and-white negative films and motion picture negative films.
"Toning" refers to a process wherein the normal neutral gray black-and-white image obtained in the conventional photochemical process to a stable form that is not oxidizable. In addition, the color of the image may be changed. In some toning processes, the metallic silver image obtained after development is converted to a silver sulfide image to produce what are commonly known as "sepia" prints that range in color from yellowish brown to a color approaching purple.
Sulfide toning methods may be either direct in which the silver image is converted at once into the silver sulfide image, or indirect in which two steps are required. Thus, the indirect method requires bleaching metallic silver to silver halide (such as silver bromide), and then converting the silver halide to silver sulfide.
One commonly used two-part sepia toning kit is available from Eastman Kodak Company as KODAK Sepia Toner Kit. The use of this kit provides rich, warm sepia images in many conventional imaged black-and-white photographic silver halide positive materials.
However, the color or tint of a sulfide-toned photographic material depends upon the sizes and structures of the silver halide grains used, as well as the compositions of those grains and the addenda used in modern black-and-white emulsions. In addition, the type of exposure and photographic processing (for example, the development step) of the imaged materials can have an effect on the eventual toned imaged. For example, compactness of the developed silver surface area or remaining emulsion addenda may promote an undesirable image.
Toned images may be identified as "cold" or "warm" depending upon where the toned image falls within the conventional CIE color scale using a* and b* values (Commission Internationale de l'Eclairage). A "cold" tone would be an image that is on the bluish side of neutral (that is negative b*), and a "warm" tone would be an image that is on the yellow or positive b* (and partly red or positive a*) side of neutral. Methods for obtaining "cold" toned images are described for example in U.S. Pat. No. 2,607,686 (Current), U.S. Pat. No. 5,037,727 (McLean) and U.S. Pat. No. 5,688,635 (Parker et al).
As black-and-white photographic silver halide positive materials have been redesigned in recent years, for example, to have different silver halide grain compositions and sizes, and other components have been added to the silver halide emulsions, the effect of conventional toning solutions has also changed. The conventional sepia toning compositions do not always provide the desired color shift, especially to the "warm" side of neutral. Moreover, the known toning compositions do not always provide the image stability that is desired. In other words, the black-and-white images may not be sufficiently stabilized using current toning products to provide long-term image quality (metallic silver could remain after toning that was not converted to a silver salt that may be susceptible to oxidation).
Thus, there is a need in the industry for an improved means for providing "warm" sepia toned images with a greater variety of silver halide photographic positive materials, and to provide images with improved stability.